American farmers, as the recent Super Bowl ad showed, are just simple, hard-working folk, scratching a living from the land. Right? Except for the hard-working part, don’t believe it. Farmers are capitalists, just as much as their cousins in the big city. Case in point: as the price of crops like corn and soybeans has risen considerably in recent years—thanks to increased demand, both for food and as feedstocks for biofuel—farmers have been planting more of them. Supply and demand–it’s Econ 101 at its purest. (With the exception of the billions upon billions of dollars worth of market-distorting subsidies that are part of the agriculture sector. But that’s grist for another post.)

If a farmer wants to increase the amount of grain they produce, he really only has a couple of options. He can try to squeeze more crop out of the land he’s already farming, which is something American farmers have been pretty good at. (Corn yields per acre have increased by more than two and a half times since 1960.) Or he can expand the amount of land that he’s farming, by converting or buying non-farmed land and putting it into cultivation. Land, after all, is the raw material of agriculture, like steel and rubber are the raw materials of a car. More land means more crops. Add in the fact that funding has been declining for the government’s Conservation Reserve Program, which pays farmers to protect wildlife by keeping land uncultivated.

According to a new study in the Proceedings of the National Academy of Sciences, that’s exactly what’s happened to the western end of the great U.S. corn belt. Researchers from South Dakota State University crunched the numbers and found that 1.3 million acres of grassland in disappeared between 2006 and 2011 in North Dakota, South Dakota, Nebraska, Iowa and Minnesota. The grassland was converted to cropland, as farmers expanded their territory in an effort to cash in—and I mean that in a totally non-pejorative sense—on the boom in crop prices. In South Dakota and Iowa, as much as 5% of grassland per year was converted to farmland.

As lead author Christopher Wright told NPR, “This is kind of the worst-kept secret in the Northern Plains.” You can literally see the land being converted by the plow. But as the study goes onto state, turning grassland to cropland can have negative consequences for the larger environment:

For instance, it’s bad news for wildlife, because corn fields are much less inviting habitat for a wide range of wild creatures, from ground-nesting birds to insects, including bees. Corn and soybean fields are increasingly encroaching into the Prairie Pothole region of the Dakotas and Minnesota, the most important breeding habitat for waterfowl in North America.

Farmland—especially American farmland—is great for producing food that supports human beings. But it’s not so great for supporting other species—at least compared to native grasslands. (And it’s not just a problem for the U.S. The world may need to feed some 9 billion people by mid-century, which could mean doubling food production from current levels. Farming already covers nearly 40% of the planet’s land area. If that doubling is achieved by significantly expanding the amount of the planet under the plow, well, there really won’t be much wilderness left to save. We’ll be living on Planet Corn Belt.